1. Field of the Invention
The present invention relates to a solid oxide fuel cell that excels in output performance. Specifically, the present invention relates to a solid oxide fuel cell having an electrolyte membrane free of gas permeability that excels in oxygen-ionic conductivity, and having a high output performance even at a power-generating temperature of 900.degree. C. or below.
2. Description of the Related Art
As an electrolyte membrane for a conventional solid oxide fuel cell, a layer composed of yttria-doped zirconia (hereafter referred to as YSZ) has been proposed. (For example, see Patent Document 1.) Since YSZ excels in sintering properties, it is possible to easily fabricate an electrolyte membrane free of gas permeability; however, there has been a problem that the output performance of a solid oxide fuel cell using this material for the electrolyte membrane is deteriorated because the oxygen-ionic conductivity of the electrolyte membrane is decreased at low temperatures of 900.degree. C. or below, and the reaction of Equation (1) occurring between the air electrode and the electrolyte membrane cannot be efficiently promoted.½O2+2e−→O2−  (1)
The electrolyte membrane free of gas permeability used herein is evaluated by the gas permeability of the gas permeated between one side and the opposite side of the electrolyte membrane when pressure difference is created between them, and has a gas permeability Q≦2.8×10−9 ms−1 Pa−1 (more preferably Q≦2.8×10−10 ms −1 Pa−1).
As an electrolyte membrane for a solid oxide fuel cell, a layer composed of scandia-doped zirconia (hereafter referred to as SSZ) has been proposed. (For example, see Patent Document 2.) Since SSZ has a higher oxygen-ionic conductivity than YSZ, it is expected that the output performance of a solid oxide fuel cell is elevated if this material is adopted as the electrolyte membrane. However, although the present inventors fabricated an electrolyte membrane using the material described in Patent Document 2, as a result of the test conducted by the present inventors, it was difficult to fabricate an electrolyte membrane free of gas permeability.
As an electrolyte membrane for a solid oxide fuel cell, a layer composed of ceria doped with samaria, gadolinia or the like has been proposed. (For example, see Patent Documents 3 and 4.) Since the material proposed in Patent Documents 3 and 4 has electronic conductivity in a fuel-gas environment of the solid oxide fuel cell, there has been a problem that the output performance is deteriorated if the electrolyte membrane is composed of the proposed materials only.
As an electrolyte membrane for a solid oxide fuel cell, a layer composed of lanthanum gallate has been proposed. (For example, see Patent Documents 5 and 6.) When a manganese-containing oxide, such as lanthanum manganite, is adopted as the air electrode of the solid oxide fuel cell, electronic conductivity is produced in the material proposed in Patent Documents 5 and 6 due to diffusion of manganese components, and there has been a problem that the output performance is deteriorated if the electrolyte membrane is composed of the proposed materials only.
[Patent Document 1] Japanese Patent Application Publication No. 10-158894 (p. 1-6, FIGS. 1-12)
[Patent Document 2] Japanese Patent Application Publication No. 7-6774 (p. 1-5, FIGS. 1-5)
[Patent Document 3] Japanese Patent Application Publication No. 11-273451 (p. 1-8, FIGS. 1-5)
[Patent Document 4] Japanese Patent No. 2813355 (p. 1-5, FIGS. 1-5)
[Patent Document 5] Japanese Patent Application Publication No. 2002-15756 (p. 1-9, FIGS. 1-9)
[Patent Document 6] Japanese Patent Application Publication No. 11-335164 (p. 1-12, FIGS. 1-12)